Solar Wind Sputtering Rates of Small Bodies and Ion Mass Spectrometry Detection of Secondary Ions

Abstract

Solar wind interactions with the surfaces of asteroids and small moons eject atoms and molecules from the uppermost several nanometers of regolith grains through a process called sputtering. A small fraction of the sputtered species, called secondary ions, leave the surface in an ionized state, and these are diagnostic of the surface composition. Detection of secondary ions using ion mass spectrometry (IMS) provides a powerful method of analysis due to low backgrounds and high instrument sensitivities. However, the sputtered secondary ion yield and the atomic composition of the surface are not 1‐to‐1 correlated. Thus, relative yield fractions based on experimental measurements are needed to convert measured spectra to surface composition. Here available experimental results are combined with computationally derived solar wind sputtering yields to estimate secondary ion fluxes from asteroid‐sized bodies in the solar system. The Monte Carlo simulation code SDTrimSP is used to estimate the total sputtering yield due to solar wind ion bombardment for a diverse suite of meteorite and lunar soil compositions. Experimentally measured relative secondary ion yields are analyzed to determine the abundance of refractory species (Mg+, Al+, Ca+, and Fe+) relative to Si+, and it is shown that relative abundances indicate whether a body is primitive or has undergone significant geologic reprocessing. Finally, estimates of the sputtered secondary ion fluxes are used to determine the IMS sensitivity required to adequately resolve major element ratios for nominal orbital geometries.

Phobos Surface Sputtering as Inferred From MAVEN Ion Observations

“…The determination of sputtering yields of ions with monoatomic and complex material is an active area of research, and most available estimates rely on Monte Carlo simulations conducted with either the SRIM/TRIM (Ziegler et al, ) or SDTrimSP (Mutzke et al, ) softwares, as experimental databases keep growing but remain limited (Schaible et al, ; Szabo et al, , and references therein). In this article, we use the total sputtering yields for impacting H + , He 2+ , and O + ions given by Figure c of Poppe and Curry (), computed by Biersack and Eckstein () for a monoatomic Ni target and very similar to estimates of Matsunami et al () for Ni or Fe atoms.…”

“…Journal of Geophysical Research: Planets keep growing but remain limited (Schaible et al, 2017;Szabo et al, 2018, and references therein). In this article, we use the total sputtering yields for impacting H + , He 2+ , and O + ions given by Figure 1 c of Poppe and Curry (2014), computed by Biersack and Eckstein (1984) for a monoatomic Ni target and very similar to estimates of Matsunami et al (1984) for Ni or Fe atoms.…”

“…Schaible et al (2017) computed energy-dependent sputtering yields of H + and He 2+ on SiO 2 and Al 2 O 3 targets. As for KREEP, the sputtering yields that we use here are within a factor of 2 in agreement with the yields published by Schaible et al (2017).…”

“…At this specific kinetic energy, one can see that the sputtering yields that we use are in agreement with KREEP sputtering yield estimates of Barghouty et al () within less than a factor of 2. Schaible et al () computed energy‐dependent sputtering yields of H + and He 2+ on SiO 2 and Al 2 O 3 targets. As for KREEP, the sputtering yields that we use here are within a factor of 2 in agreement with the yields published by Schaible et al ().…”

In Situ Observations of Ions and Magnetic Field Around Phobos: The Mass Spectrum Analyzer (MSA) for the Martian Moons eXploration (MMX) Mission

“…The Mars-facing hemisphere is exposed to predominantly oxygen ions while the anti-Mars hemisphere is exposed to solar wind (Nenon et al, 2019), and thus distinguishing between magnetospheric oxygen and oxygen from water ice is critical. It was shown that classification of targeted small bodies can be carried out by measuring the flux ratios of refractory ions such as Mg + , Ca + , and Fe + to Si + even with an accuracy of ~50% (Schaible et al 2017). Therefore, MSA measurements of secondary ions sputtered from Phobos by the solar wind (especially refractory ions) will allow us to discriminate between the formation models.…”